1. Field of the Invention
This invention generally relates to electrochemical cells and, more particularly, to a method of fabrication and associated cyanometallate cathode battery with improved capacity.
2. Description of the Related Art
The rechargeable lithium ion battery (LIB) has triggered the portable electronic devices revolution due to its high power density, long cycling life, and environmental compatibility. The rechargeable LIB consists of a cathode (positive electrode) and an anode (negative electrode), separated by a Li+-ion permeable membrane. A solution or polymer containing lithium-ions is also used in the battery so that Li+-ions can “rock” back and forth between the positive and negative electrode freely. The positive materials are typically transition-metal oxides such as lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), lithium iron phosphate (LiFePO4), and their derivatives. Lithium-ions can move in their interstitial space freely and reversibly. The negative electrode materials can use lithium-metal, alloys, and carbonaceous materials. During discharge, Li+-ions are extracted from the negative electrode and inserted into the positive electrode, in the meantime, electrons pass through an external circuit from the negative electrode to the positive electrode and generate electric power. During a charge, ions and electrons move along the reverse direction and go back to their original places.
Although LIBs have been successfully used, the conflict between lithium demand and its scarcity surges its cost, which hinders the further application of lithium-ion batteries on a large scale. Therefore, a low-cost rechargeable battery is urgently needed as an alternative to expensive LIBs. Under the circumstance, sodium-ion batteries are attracting more attention because sodium has very similar properties to lithium, but a cheaper cost. Like lithium-ion batteries, sodium-ion batteries need Na+-host materials as their electrode. Much effort has been expended to directly duplicate the Li+-host structures, using Na+-host electrode materials for the sodium-ion batteries. For example, NaCoO2, NaMnO2, NaCrO2 and Na0.85Li0.17Ni0.21Mn0.64O2, all having a layered-structure similar to LiCoO2, have been developed for sodium-ion batteries. Similarly, Co3O4 with a Spinel structure, Na3V2(PO4)3 with a NASICON structure, and NaFePO4 with an Olivine structure have been employed in sodium batteries. In addition, sodium fluorophosphates, such as Na2PO4F, NaVPO4F and Na1.5VOPO4F0.5, have also used as the positive electrode in sodium batteries.
However, it is impractical to copy the structures of Li+-host compounds for Na+ or K+-host compounds. Sodium and potassium ions are much larger than lithium ions, and severely distort the structure of the Li+-host compounds. Thus, it is very important for the advancement of sodium/potassium-ion batteries to develop new Na+/K+-host materials with large interstitial spaces in which sodium/potassium-ions can easily and reversibly move. Na+/K+-ions have been observed to intercalate into metal cyanide compounds. Transition metal hexacyanoferrates (TMHCFs) with large interstitial spaces have been investigated as cathode materials for rechargeable lithium-ion batteries [1,2], sodium-ion batteries [3,4], and potassium-ion batteries [5]. With an aqueous electrolyte containing proper alkali-ions or ammonium-ions, copper and nickel hexacyanoferrates [(Cu,Ni)-HCFs] demonstrated robust cycling life with 83% capacity retention after 40,000 cycles at a charge/discharge current of 17C [6-8]. In spite of this, the materials demonstrated low capacities and energy densities because (1) only one sodium-ion can be inserted/extracted into/from per Cu-HCF or Ni-HCF formula, and (2) these TM-HCFs electrodes must be operated below 1.23 V due to the water electrochemical window. The electrochemical window of a substance is the voltage range between which the substance is neither oxidized nor reduced. This range is important for the efficiency of an electrode, and once out of this range, water becomes electrolyzed, spoiling the electrical energy intended for another electrochemical reaction.
In order to compensate for such shortcomings, manganese hexacyanoferrate (Mn-HCF) and iron hexacyanoferrate (Fe-HCF) have been used as cathode materials in a non-aqueous electrolyte [9, 10]. When assembled with a sodium-metal anode, Mn-HCF and Fe-HCF electrodes cycled between 2.0V and 4.2 V delivered capacities of ˜110 mAh/g.
FIG. 1 is a diagram depicting the crystal structure of a metal hexacyanometallate (MHCM) (prior art). The aforementioned TMHCFs can be categorized into a more general group of MHCMs that have the open framework as shown. MHCMs can be expressed as AXM1YM2Z(CN)N. MH2O, where A can be selected from, but not limited to alkali and alkaline metals, and M1 and M2 are transition metals such as titanium (Ti), vanadium (V), chrome (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), calcium (Ca), magnesium (Mg), etc. M1 and M2 can be the same or a different metal. The ratio (X:N) of M1 and M2 is an arbitrary number, depending on the materials used. In addition, various amounts of water (H2O) can occupy in interstitial or lattice positions of MHCMs.
The large interstitial spaces of MHCMs accommodate many kinds of metal ions, which could be used as electrode materials in many kinds of metal-ion batteries. The electrochemical reactions in the batteries can be expressed as follows.
at the cathode:
            A      x        ⁢    M    ⁢                  ⁢                            1          y                ⁡                  [                      M            ⁢                                                  ⁢            2            ⁢                                          (                CN                )                            z                                ]                    n        ⁢          mH      2        ⁢    O    ⁢      ⇄    Discharge    Charge    ⁢            xA              a        +              +                  A        x            ⁢      M      ⁢                          ⁢                                    1            y                    ⁡                      [                          M              ⁢                                                          ⁢              2              ⁢                                                (                  CN                  )                                z                                      ]                          n            ⁢              mH        2            ⁢      O        +          xae      -      
at the anode:
            A              a        +              +          ae      -        +    ∅    ⁢      ⇄    Discharge    Charge    ⁢      A    -    ∅  
In MHCM batteries, metal “A” can be simply act as the counter electrode (anode). However, the use of a metal anode inevitably causes dendrite growth during charge/discharge. Metal dendrites may penetrate through the electronic insulators between anode and cathode to short the batteries. As a result, non-metal materials, alloys, and intercalation compounds, indicated as Ø in above electrochemical reactions, have been developed to replace metal electrodes. In lithium-ion batteries for example, graphite has been used as a substitute for a lithium-metal anode.
To obtain high voltages with MHCM batteries, the anode electrodes are expected to have a low potential around which the electrolyte may be reduced. Meanwhile, passivating layers, nominally solid electrolyte interface (SEI) layers, form on the anode materials to prevent further electrolyte reactions. Simultaneously intercalating metal ions from cathode are exhausted irreversibly. If these ions cannot be replenished when non-intercalating ion loaded anode electrodes are used, the batteries demonstrate low energy densities.
The use of non-intercalating ion loaded anode electrodes presents another problem. If the intercalating ions are not primarily stored in the cathode, then it is impossible to assemble battery with the cathode and non-intercalating ion loaded anode. For example, capacity problems exist for a battery with a Berlin green (Fe2(CN)6) cathode and a hard carbon (HC) anode, even though Berlin green has been proved to be a useful cathode material in batteries.
It would be advantageous if cyanometallate cathode batteries could be fabricated in a manner that could activate the anode electrodes and form SEI layers, without exhausting intercalating ions from cathode or anode, in order to safely improve battery energy densities.    [1] V. D. Neff, “Some Performance Characteristics of a Prussian Blue Battery”, Journal of Electrochemical Society 1985, 132, 1382-1384.    [2] N. Imanishi, T. Morikawa, J. Kondo, Y. Takeda, O. Yamamoto, N. Kinugasa, and T. Yamagishi, “Lithium Intercalation Behavior into Iron Cyanide Complex as Positive Electrode of Lithium Secondary Battery”, Journal of Power Sources 1999, 79, 215-219.    [3] Y. Lu, L. Wang, J. Cheng, and J. B. Goodenough, “Prussian Blue: a New Framework for Sodium Batteries”, Chemistry Communications 2012, 48, 6544-6546.    [4] L. Wang, Y. Lu, J. Liu, M. Xu, J. Cheng, D. Zhang, and J. B. Goodenough, “A Superior Low-Cost Cathode for a Na-ion Battery”, Angewandte Chemie International Edition 2013, 52, 1964-1967.    [5] A. Eftekhari, “Potassium Secondary Cell Based on Prussian Blue Cathode”, Journal of Power Sources 2004, 126, 221-228.    [6] C. D. Wessells, R. A. Huggins, and Y. Cui, “Copper Hexacyanoferrate Battery Electrodes with Long Cycle Life and High Power”, Nature Communications 2011, 2, Article number: 550.    [7] C. D. Wessells, S. V. Peddada, R. A. Huggins, and Y. Cui, “Nickel Hexacyanoferrate Nanoparticle Electrodes for Aqueous Sodium and Potassium Ion Batteries”, Nano Letters 2011, 11, 5421-5425.    [8] C. D. Wessells, S. V. Peddada, M. T. McDowell, R. A. Huggins, and Y. Cui, “The Effect of Insertion Species on Nanostructured Open Framework Hexacyanoferrate Battery Electrodes”, Journal of the Electrochemical Society 2012, 159, A98-A103.    [9] T. Matsuda, M. Takachi, and Y. Moritomo, “A Sodium Manganese Ferrocyanide Thin Film for Na-ion Batteries”, Chemical Communications 2013, 49, 2750-2752.    [10] S-H. Yu, M. Shokouhimehr, T. Hyeon, and Y-E. Sung, “Iron Hexacyanoferrate Nanoparticles as Cathode Materials for Lithium and Sodium Rechargeable Batteries”, ECS Electrochemistry Letters 2013, 2, A39-A41.